I think that the reduction in lead is far too late, if you figure that it's cumulative exposure over someone's lifetime, not short-term (which I have not looked up, but would expect to be the case).

googles to sanity check

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5801257/

In this cohort study of 553 New Zealanders observed for 38 years, lead exposure in childhood was weakly associated with official criminal conviction and self-reported offending from ages 15 to 38 years. Lead exposure was not associated with the consequential offending outcomes of a greater variety of offenses, conviction, recidivism, or violence.

Yeah, so it's a childhood thing. You'd be talking about on the order of maybe a 20 year delay until a reduction in exposure translates into peak potential serial killer period.

Also, for stuff like lead paint, it's gonna be around for decades, gets kicked up over time, so it takes an even longer time for regulations to go have an effect, and that effect is very spread out, whereas this is a pretty sharp increase and decrease.

https://en.wikipedia.org/wiki/Lead-based_paint_in_the_United_States

In 1971, Congress banned the use of lead-based paint in residential projects (including residential structures and environments) constructed by, or with the assistance of, the federal government.[3] The Consumer Product Safety Commission followed with implementing regulations, effective in 1978.[4] Additional regulations regarding lead abatement, testing and related issues have been issued by the Environmental Protection Agency (EPA).

I'd -- without digging up numbers -- guess that halting leaded gasoline probably had the most-immediate impact on lead in the air, since burning leaded gasoline is gonna put it straight into the air.

https://en.wikipedia.org/wiki/Gasoline#Lead_Replacement_Petrol

In the U.S., the Environmental Protection Agency issued regulations to reduce the lead content of leaded gasoline over a series of annual phases, scheduled to begin in 1973 but delayed by court appeals until 1976.

If something were gonna happen in the 1970s to reduce the rate of serial killing, to be a relevant input, it'd have to be something that had a major immediate effect rather than a long-term developmental effect.